The long-term goal of our research is to understand the cellular and molecular basis for B cell memory, an essential component of immunity to most pathogens. We have identified a population of IgM-positive memory cells during bacterial infection that, following antigen challenge, give rise to a robust IgG response. The IgM memory cells constitute a novel and important subset of long-lived memory B cells that may provide recall immunity to variant pathogens not recognized by classical high-affinity class-switched memory B cells. We will use our experimental mouse model of human ehrlichiosis to investigate important questions regarding the development and differentiation of IgM memory cells. Our new preliminary data indicate that, following secondary infection, IgM memory cells 1) are responsible for earlier production of IgM, via the generation of long-term IgM-producing cells; 2) undergo further differentiation and diversification within germinal centers; 3) are a source of new class-switched memory cells; and 4) are maintained, as stem cells, by a process of self- renewal. We will address these preliminary findings in greater depth in Aim 1, by first comparing the differentiation and diversification of IgM and switched memory cells following secondary challenge infection, and by evaluating their capacity for protection. We will also formally test th novel hypothesis that the IgM memory cells have the capacity to self-renew and reconstitute a complete spectrum of memory B cells. In Aim 2, we address how the IgM memory cells develop by identifying the B cell subsets which give rise to the memory cells, by testing whether these cells require germinal centers to complete their development, and by investigating how a type of CD4 T cell known as a follicular T cell contributes to IgM memory cell ontogeny. Thus, the proposed research will provide fundamental knowledge of immunological memory, but will also contribute to the design of vaccines and therapies that can elicit durable humoral memory.